Traditional percussion drilling of wells and boreholes in the earth with pneumatically-operated downhole hammers is a well-developed art. High penetration rates in hard formations make this an attractive drilling method in mining, quarry, construction, water well, oil well and geophysical applications. A typical system employs a string of conventional drill pipe and an air hammer which drives a percussion bit. Air from a surface compressor is directed down the pipe to operate the hammer, and the entire string is usually rotated. The air exhausts through the bit at the bottom of the hole and returns to the surface through the annular space between the hole wall and the drill string (the borehole annulus).
A major problem with such a system is the need for relatively high air compressor capacity. In order to ensure proper bailing of cuttings from the bottom of the hole, upward annular air velocities in the borehole annulus of at least 4,000 feet per minute are usually recommended. Such a flow is expensive and difficult to achieve, particularly where the bit is of appreciably larger diameter than the drill string, resulting in a large borehole annulus. Other problems include lost circulation, dust control, sample contamination, formation water, sticking and the like.
Attempts to solve these and other problems have heretofore been proposed involving the use of dual concentric drill pipe in conjunction with pneumatic hammers. In such systems, the air for operating the hammer is directed down the annular conduit (the pipe annulus) defined between the inner and outer tubes of the pipe string, and at least a portion of the exhaust air and entrained cuttings is directed to the surface through the central conduit defined by the inner tube of the pipe string. One such system is proposed in U.S. Pat. No. 3,299,971. The patentee suggests, however, that pressurized fluid be introduced into the borehole annulus to insure movement of fluid down the hole, around the bit and up the inner tube. A similar system is proposed in U.S. Pat. No. 3,871,486. There the patentees suggest that a major portion of the exhaust air be directed up the borehole annulus. Thus neither system takes full advantage of the enclosed circulation available with dual tube pipe. Moreover, both patents disclose and depend upon "hollow" or axial flow hammers and bits suitable for cutting cores. Such hammers do not appear to be readily available commercially--probably due to technical problems of design and manufacture--and certainly are not stock items.
Another approach is proposed in U.S. Pat. Nos. 3,667,555 and 3,747,698, which utilize conventional air hammers which are widely available in various sizes and configurations. In order to adapt such a non-axial flow hammer for use with dual tube pipe, the patentees disclose cross-over subs which direct drilling fluid from the pipe annulus to operate the hammer and which direct exhaust air and cuttings into the inner tube. The former patent also teaches that a portion of the drilling fluid from the annular conduit should be injected into the borehole annulus uphole from the sub.
These systems do not completely overcome the drawbacks of the prior art. For one thing, the borehole annulus around the hammer itself remains large, making it difficult to achieve the annular air velocity required to insure proper removal of cuttings. Moreover, there is a tendency toward sticking due to small cuttings or "fines" lodging in the borehole annulus or bypassing the sub entirely. Finally, the injection of a portion of the drilling fluid into the borehole annulus above the sub to maintain a constant uphole flow in the borehole is inefficient.